API: libcrmcommon: New PCMK_XA_TARGET_ROLE string constant

Not to be confused with PCMK_META_TARGET_ROLE. They're used in different
contexts (PCMK_XA_TARGET_ROLE is used for API output), and they're
slightly different literal strings (underscore vs. hyphen).

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>